The present invention relates to an improvement in an electronic watch which has driving power automatic control means for a stepping motor for the purpose of reducing the power consumption of the stepping motor to provide an electronic watch which can be operated stably at the end of its battery life, and operated with lower power consumption.
The display mechanism of an conventional crystal watch of the analog type now in use is constructed generally as shown in FIG. 1. The output of the motor consisting of a stator 1, a coil 7 and a rotor 6 is transmitted to different wheels 2, 3, 4 and 5 and a second pointer, a minute pointer, an hour pointer as well as a calender are driven by the wheels together with other wheels not shown. FIG. 2 shows a circuit construction of the conventional electronic watch. A signal of about 32 KHz from an oscillating circuit 10 is converted into a second signal by a frequency dividing circuit 11. The second signal is further converted into a signal having either a 1.8 msec or 2 sec period by pulse combining circuit 12. To input terminals 15, 16 of drive inverters 13a and 13b is applied a signal having the same pulse period and width but dephased by one second, so that an inverted pulse alternating every one second is applied to a coil 14. The rotor 16, magnetized in two poles, begins to rotate in one direction. The coil current wave shape with this case is shown in FIG. 3.
In the meantime, the drive pulse width in the electronic watch according to the conventional art, i.e., 7.8 msec in the conventional watch, is designed in such a way that the factors such as coil resistance, number of turns, and size of the stepping motor are suitably selected so as to drive the stepping motor in a stable condition under worst care conditions, such as when the load on the wheels increases the watch is placed in a magnetic field, the internal resistance of the watch battery strikingly increases due to very low temperature, or the battery voltage lowers because of exhaustion of the battery. One drawback of conventional watches is that when a large torque is not necessary, the drive pulses of fixed pulse width cause excess consumption of the battery.
To overcome such drawbacks as described above, a method for driving a stepping motor has been proposed recently wherein detecting means for detecting the operating condition of the stepping motor is provided so as to change the drive power (generally, the driving pulse width) continuously or steppingwise at minimum power.